1 Introduction
Numerous aphid species are economically important plant pests that feed on plant sap. Many plant-feeding aphids can also transmit plant viruses. Around 100 out of approximately 5,000 known aphid species are significant agricultural pests due to their feeding damages and/or disease transmission (Blackman & Eastop, 2020). Currently, studies on aphid genomes have mainly focused on the subfamily Aphidinae (International Aphid Genomics Consortium, 2010; Li et al., 2019; Mathers, 2020; Mathers et al., 2017; Mathers, Mugford, et al., 2020; Mathers, Wouters, et al., 2020; Nicholson et al., 2015; Thorpe et al., 2018; Wenger et al., 2016). Genome sequencing on species from other subfamilies that are distantly related to Aphidinae is relatively limited (Julca et al., 2020; Biallo et al., 2020). Unlike most free-living aphids, galling aphids can induce gall formation on their primary host plants and then live in galls. Galling aphids may are ideal models to study unique ecological and behavioral phenomena underlying insect-plant interactions and their coevolution (Moran, 1989; Wool, 2004). So far, only two galling aphids have been sequenced and assembled (Eriosoma lanigerum and Hormaphis cornu ). The aphidE. lanigerum often causes bark deformation and cancer-like swelling on the roots, trunk or brunches of apple, and sometimes induces the formation of leaf-rosette galls on American elm (Ulmus americana ) (Blackman and Eastop, 2020). The aphid, H. cornu , induces a gall on the underside of leaves of witch hazel,Hamamelis virginiana (Kurosu et al., 1992). The galls induced byE. lanigerum and H. cornu are quite different from the completely closed galls induced by Schlechtendalia chinensis , which has peculiar strategies to adapt to a closed environment that has extremely high levels of CO2 honeydew, and other aphid metabolites (Chen et al., 2020).
The horned gall aphid,S. chinensis (Hemiptera: Aphididae: Eriosomatinae: Fordini), is one of the most economically valuable insects. Gallnuts induced by the aphids are valuable for medicinal purposes and in chemical industries. The components in gallnuts, such as tannins, are important gradients for producing inks, wine, food, cosmetic antioxidants, and animal feed. High levels of tannins (50- 70%) have been found in horned galls (Zhang, Tang, & Cheng, 2008). The annual yield of gallnuts in China is 8,000-10,000 tons, accounting for over 90% of the total yield worldwide (Zhang, Tang, & Cheng, 2008).
S. chinensis has a complex life cycle involving both sexual and asexual reproduction stages with a host alternation between the Chinese sumac (Rhus chinensis , Anacardiaceae) and mosses of the genus (Plagiomnium spp.,Mniaceae). In this holocyclic life cycle, a fundatrix produced by a mated female crawls along the trunk and feeds on a new leaf, where it induces the formation of a horned gall. The fundatrix can produce wingless fundatrigeniae in galls via parthenogenesis. In autumn, wingless fundatrigeniae will produce winged fundatrigeniae named autumn migrants. When galls become mature and burst open, the alate autumn migrants will fly to nearby mosses and produce nymphs for overwintering. In the following spring, nymphs on mosses will develop into spring winged migrants, which then fly back to the primary host, R. chinensisand produce both female and male offspring called sexuales. After mating, each female reproduces only one fundatrix, starting the cycle again (Figure 1) (Zhang, Qiao, Zhong & Zhang, 1999; Blackman and Eastop, 2020). This representing an unusual life cycle with comprising various morphologically distinct aphid forms at different stages, and its evolution was likely driven by the adaptation to different environmental conditions. Unlike most free-living aphids from the Aphidinae taxon, galling aphids exhibit diverse biological characteristics. For example, most galling aphid species do not seriously affect the health of their host plants. In some cases, the galls are thought to be beneficial to host plants (Chen et al., 2020).
For S. chinensis , the complexities in its developmental process and the structure of its induced galls imply that it may possess unique gene sets that regulate its development and manipulate its host plants (Takeda et al., 2019; Hirano et al., 2020). The molecular mechanisms underlying its complex life cycle remain largely unknown. Galls are produced through the insect-driven dramatic reprogramming of plant cell biology. Previous studies have shown that gall induction is highly species-specific, and that different galling insects deliver unique sets of effectors into plant tissues, resulting in gall formation (Zhao et al., 2015; Aljbory et al., 2018). The underlying mechanisms of the parasitic ability of galling aphids on host plants via apparently harmless galls remain unknown so far. To understand the genetic basis of the complex lifestyle, a high-quality chromosome-level genome assembly of S. chinensis accomplished, representing the first genome sequence of aphids that induces the formation of completely closed galls. Phylogenetic relationship between S. chinensis and closely related species was analyzed to better understand the unique biological characteristics of S. chinensis .